1. Field of the Invention
This invention relates to the recovery of material fines in slurry or sludge form to an agglomerate of usable size and moisture content for reuse in a process of manufacture, such as a fuel or metallurgical fluxing or conditioning agent, for example.
While the disclosure herein will be primarily related to the recovery of coal silt or fines from a slurry, it is to be recognized by those skilled in the art that the present invention is applicable to the recovery of any material fines found in slurry or sludge form. For example, it is often desirable to recover material fines from flue dust slurries created as a by-product in any of the metallurgical refining processes for reuse in agglomerated form as fluxing agents or slag conditioning agents, or as basic metallurgical ingredients in the production or refining of metals or other products. As merely one example of application, reference is made to U.S. Pat. No. 3,799,762 (assigned to the same assignee of record) wherein it is taught that waste or by-product dust from the manufacture of ferro-manganese may be utilized in a fluorspar slag conditioner. Such by-products may be recovered in accordance with the teachings of the present invention. For example, fluorspar-bearing sludge mixtures may be treated and dried successfully with the application of the teachings of this invention. The same would hold true for recovery of flue dust from a BOF steel refining process, for example, which are also generally found in the form of a slurry as a result of the use of water in scrubbers or other cleaning devices.
2. Discussion of the Prior Art
For convenience, discussion of the prior art will be made with particular reference to the recovery of coal silt.
In coal preparation plants, a considerable amount of fine coal is deposited in waste ponds known as settling or slurry ponds. These ponds are created by pumping the waste coal bearing slurries into ponds where the solids settle out and the clarified water is allowed to decant off. These waste slurries may originate from a number of sources or stages in the preparation and handling of the coal, such as from underflow from thickening operations, from water used in concentrating or separating tables, from overflow from chance cone systems, or from wet scrubbings from dust collectors utilized on drying or handling operations.
Throughout the country, there are many active and dormant settling ponds containing valuable coal fines. Although the chemical makeup of the fine coal in the ponds varies considerably from pond to pond as noted by H. Charmbury in Characteristics of Coal Preparation Slurries, Mining Engineering, January, 1960, page 49, many ponds contain coal which is of recoverable value. The coal in these ponds is generally in the form of a wet, cakey sludge or slurry containing a considerable amount of moisture, usually about 30% to 40%. The moisture content may vary considerably from pond to pond. In a survey made in the Commonwealth of Pennsylvania as noted in the aforementioned article, for example, the ash content of a dry basis may vary from approximately 10% to 40% and the sulfur content from approximately 1% to 3%. In addition, the settling ponds exist from both bituminous and anthracite preparation plants, and thus the BTU value of the coal contained in the ponds may vary substantially for all of the above reasons from pond to pond.
However, once it has been determined that the coal silt of a particular pond is of sufficient quality to warrant recovery, then the prior art basically teaches two possible ways to economically recover coal sludge from the settling ponds. The first involves drying the sludge down to from 7% to 12% moisture and reblending this material in small percentages into a prepared coal product. The other approach involves agglomerating the fines and using the agglomerate as a fuel source. The prior art of agglomerating coal fines is discussed at length in The Application of the Pelletizing Process to the U.S. Coal Industry, by P. T. Luckie and T. S. Spicer, 1965 Proceedings of the International Briquetting Association at page 61.
It may be noted in this article that a number of methods for agglomeration of coal fines have been proposed, including briquetting of coal fines, extrusion of coal silt and pelletizing. While some encouraging results were noted, nevertheless, in all three of these agglomeration processes there are three primary problems which have heretofore not been adequately or economically resolved.
These three primary problems which have not been heretofore resolved to an acceptable state, involved in the recovery of coal sludge from settling ponds are as follows: (1) handling the wet, cakey material; (2) drying the wet sludge down to an acceptable level; and (3) controlling coal dust problems and hazards associated with fine dry products. It will be noted that all three of these problems are encountered with the recovery of not only coal sludge from settling ponds, but in the recovery of any material sludge in related arts as previously explained.
The first two problems, handling and drying the coal sludge, may be considered together. In order to dry a wet, cakey material such as a coal sludge or a metallurgical by-product sludge, it is necessary to apply heat to the sludge and to move and expose enough sludge surface to allow the water to evaporate. Due to the sticky, cakey nature of the sludge, for example coal sludge at about 30% moisture as it comes from the ponds, this operation of handling and drying has proved to be extremely difficult to accomplish with conventional drying methods. For example, rotary kilns are not satisfactory for drying coal sludge, as the sludge will stick or ring up along the sides of the kiln thereby preventing the necessary exposure of free particle surfaces for drying. The material that does not break away from the sides of the kiln is over exposed to the heat and will thereby become excessively dry creating both a dangerous dust hazard and a non-uniform final product in regard to size and moisture content. In addition, a kiln is also large, cumbersome, expensive and somewhat difficult to regulate with regard to the manufacture of a controlled product.
Two prior art drying techniques are presently in use for drying coal sludge. The first involves the use of a vibratory conveyor which transports and fluidizes the sludge while hot air is being blown through it. This method is disclosed in Hawley Successful in Adapting System For Recovering Ultra-Fines, by R. Mason, Coal Mining and Processing, May, 1973. The chief shortcoming of this technique is that the coal sludge must first be dried to 22% moisture before it will fluidize on such a system. Thus, operations presently utilizing this system air dry the pond sludge by digging it out of the pond and turning it over from time to time in dry weather on the pond's banks until it contains less than 22% moisture. At first glance, it may be readily seen that effective bank drying is, of course, dependent upon dry weather conditions. A further shortcoming of the vibratory fluidizer system is its fuel limitation.
Ideally, when drying coal sludge, one would prefer to be able to burn coal as it is available on the site, and further in view of the fact that other fuels are now in short supply. The vibrating fluidizer consists of a vibrating screen deck with minute holes through which the hot air passes to fluidize the fine coal. Thus, burning coal directly to produce the required hot air is not possible with this system, as the ash from the coal quickly plugs the small openings or holes in the vibrating deck.
The second drying system presently in use is the "holo-flite" processor, which is a system incorporating an indirect heat exchange comprising a series of hollow rotating helical screw flights, through which a heat transfer agent is circulated, such as a hot oil. This method is disclosed in Holo- Flite Processor for Drying Coal Fines, presented to the Southwestern Virginia Coal Preparation and Engineering Society, February 18, 1974. These hot screw flights add heat to the coal sludge while gently moving and turning it over in order to release water and expose new surface area. However, the chief drawbacks with this particular approach are as follows: (1) there is no forced air or gas movement to speed and increase the drying efficiency; (2) the technique represents a large capital investment to install; (3) the parts that generally wear out and need replacement are the expensive holo-flite screws; and (4) no one at the present time manufactures a burner or system that will burn coal to create the hot oil, as the present burners known to the inventors use either gas or fuel oil.
It is a principal object of the present invention to eliminate all of the aforementioned disadvantages associated with the prior art methods of recovering sludge fines.